As one of countermeasures for the improvement of fuel efficiency of an automobile originated from a global environmental problem, a vehicle body has been decreased in weight. For this reason, there is a need to improve the strength of a metal sheet used in the automobile as much as possible. However, when the strength of the metal sheet is improved generally in order to decrease the weight of the automobile, an elongation EL or a value r (Lankford value) decreases, and hence a shape freezing property or press-formability is degraded.
In order to solve such a problem, a hot press-forming method (a so-called “hot pressing method”) that ensures a strength after a forming process is employed to manufacture a component, and the hot press-forming method is performed in a manner such that a metal sheet (blank) is heated to a predetermined temperature (for example, an austenite-phase temperature) so as to decrease the strength (that is, to facilitate the forming process), and is formed by a forming tool having a low temperature (for example, a room temperature) compared to the metal sheet (the processing target), thereby performing a shaping process and a supper-cooling heat treatment (quenching) using a temperature difference therebetween (for example, Patent Document 1).
According to such a hot pressing method, since the metal sheet is formed in a low-strength state, spring-back decreases (with a satisfactory shape freezing property), and a tensile strength becomes 1500 MPa by the quenching process. Furthermore, such a hot pressing method is called various names such as a hot forming method, a hot stamping method, a hot stamp method, and a die-quench method other than the hot pressing method.
FIG. 1 is a schematic explanatory diagram illustrating a configuration of a forming tool that is used to perform the above-described hot press-forming process. In the drawing, Reference Numeral 1 indicates a punch, Reference Numeral 2 indicates a die, Reference Numeral 3 indicates a blank holder, Reference Numeral 4 indicates a metal sheet (blank), BHF indicates a folding force, rp indicates a punch shoulder radius, rd indicates a die shoulder radius, and CL indicates a clearance between a punch and a die. Further, in these components, the punch 1 and the die 2 are respectively provided with passageways 1a and 2a through which a cooling medium (for example, water) may pass, and these members are cooled when the cooling medium passes through the passageways.
A hot press-forming facility including a press-forming machine having the above-described forming tool configuration is disclosed in, for example, Non-Patent Document 1. The facility includes a heating furnace that heats and softens a metal sheet, a device that conveys the heated metal sheet, a press-forming machine that press-forms the metal sheet, and a device that performs a trimming process (a correction process for obtaining a final shape by a laser or the like) on the forming product (see FIG. 2 below).
When a hot pressing process (for example, a deep drawing process) is performed by using such a forming tool, the forming process starts while the blank (the metal sheet) 4 is heated and softened (a direct method). That is, the metal sheet 4 is pressed into a hole (between the dies 2 of FIG. 1) of the die 2 by the punch 1 while the high-temperature metal sheet 4 is clamped between the die 2 and the blank holder 3, and is formed in a shape corresponding to the outer shape of the punch 1 while the outer diameter of the metal sheet 4 is decreased. Further, the punch and the die are cooled along with the forming process so that heat is emitted from the metal sheet 4 to the forming tool (the punch 1 and the die 2), and the punch and the die are further cooled while being held at a forming bottom dead center (a time point at which the front end of the punch is located at the deepest portion: the state shown in FIG. 1) so that the material is quenched (a die-quench process). When such a forming method is performed, a forming product of 1500 MPa with good dimensional precision may be obtained, and a forming load may be reduced compared to the case where a component having the same strength is formed by a cold forming process, so that the capacity of the pressing machine decreases. Such a forming method is also disclosed in, for example, Patent Document 2.